For me the cost of fuel was only a small part of the reason - my car mileage was low anyway.
I really needed a way of building exercise into my daily routine, and gyms just don't work for me. I was playing squash once or twice a week, but that stopped when my son was born.
What I've found, and is the reason I won't take a job that I can't cycle to unless I have no choice, is that cycling is simply a million times more enjoyable than driving. People often say that they enjoy driving, but they can't mean it - I find driving a grim experience.
I now arrive at work in a good mood, have more energy, and am much happier and fitter. I wouldn't swap it.
+1
Exactly, commuting is a way of getting the miles in for me, I am actually going longer ways than I need to most days. I think anything up to about 50 miles round trip daily is easily doable, and often the B road alternative route is shorter and certainly a lot more pleasant than driving.